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We have a large library of existing window frame tests that are
currently skipped for lacros. Many of these tests have sections that
exercise tablet mode. They will be easier to port to lacros if we
provide a test API for tablet mode.

Add a crosapi for entering and exiting tablet mode. Add a basic
smoke test, which is basically a clone of the overview mode test.

Bug: none
Test: lacros_chrome_browsertests
Change-Id: I04eb60db33b446df75a509836b2e408fc5018569
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625815
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843614}